Power modules are used for inverters and other components for driving motors. Power modules are configured such that power devices (semiconductor devices) such as IGBTs (Insulated Gate Bipolar Transistors) are mounted on insulating multilayer substrates. Specifically, such a power module employs an insulating multilayer substrate configured such that metal layers are provided on both surfaces of a ceramic layer that constitutes an insulating layer. One surface side of the insulating multilayer substrate is equipped with (mounted thereon with) a power device. The other surface side of the insulating multilayer substrate is ordinarily provided with a cooler (heat dissipator) such as a heat sink.
For next-generation semiconductor devices, SiC, GaN and Ga2O3 are expected to be used as substitute for Si, and the operating temperature will be further increased. Accordingly, more enhanced heat resistance property is required for power modules and each bonded part thereof.
In power devices, the applied voltage and the amount of electric current are large, and the amount of heat generation is accordingly also large during the operation. In order to stably operate power devices, therefore, it is necessary not only to improve the heat resistance property but also to efficiently dissipate the generated heat so that the heat is received by the insulating multilayer substrate, the cooler and other components without being accumulated.
In addition, plural layers of different materials exist in a heat-transfer path from the power devices to the cooler or the like, and thermal stresses are generated in respective bonded parts depending on the difference in coefficient of thermal expansion and the temperature difference (or temperature gradient). Unduly large thermal stresses cause the layers to readily delaminate at the bonding interfaces in accordance with the increase in the number of cooling/heating cycles acting on the power module, so that the reliability of the power module may deteriorate. Therefore, if the power module has a structure that relaxes the thermal stress acting on each bonded part, the reliability will further be improved. From such a viewpoint, various proposals are presented. For example, relevant descriptions are disclosed in the following patent literature.